1. Field of the Invention
The invention generally relates to a lens, and more particularly to a fixed-focus lens.
2. Description of Related Art
With advances in optical-electronic technologies, image apparatuses such as digital video cameras (DVC), projectors and digital cameras (DC) have been widely applied in daily life. Lens is one of core elements in the image apparatus, and configured to display images clearly on a screen or a charge coupled device (CCD). In case the image apparatus are applied in specific fields, a wide angle lens is adopted to increase a field of view (FOV). For instance, the wide angle lens is required in use of a car back-up camera, a driving assistance camera, or a security surveillance camera.
In recent years, with rapid developments of LED light source technology and digital light processing (DLP) technology, miniaturizing and lightening of the projectors have become a mainstream in product design. Further, it is a direction in which future researches can well invest as how to effectively reduce a size of an optical lens while providing both characteristics of large aperture and wide viewing angle. In addition, it is also an important consideration for the industry in development as how to reduce manufacturing costs with drastic competitions in the market while maintaining the effect of projecting large size and clear picture in limited space.
On the other hand, the conventional mini projectors on the market are prone to produce a thermal drift phenomenon because the limited heat dissipation due to restrictions of the volume of the mini projector. Therefore, in order to ensure imaging quality of the lenses, a working temperature of the mini projectors is often restricted within a narrow range.
U.S. Pat. No. 6,404,564 discloses a projection lens having two lens groups. When a distance between a projection screen end and the projection lens changes, a first lens group and a second lens group must move together to provide a projection picture with wide viewing angle. Such architecture requires a relatively complex mechanism design in which a volume of the lens is larger.
Taiwan Patent No. 1427352 discloses a fixed-focus lens. The fixed-focus lens is disposed between a magnified side and a reduced side and includes a first lens group and a second lens group arranged in sequence from the magnified side to the reduced side. The first lens group includes an aspheric lens. The second lens group having a positive refractive power is disposed between the first lens group and the reduced side, and includes an aspheric lens.
Taiwan Patent No. 1445997 discloses a fixed-focus lens. The fixed-focus lens is disposed between an object side and an image side, where the image side has an imaging surface. The fixed-focus lens includes a first lens group and a second lens group, where the first lens group is adjacent to the object side and the second lens group is adjacent to the image side.
U.S. Pat. No. 8,786,957 discloses a lens system, in which refractive powers of a third lens, a first sub lens and a second sub lens and a coefficient dn/dt satisfy a specific mathematical relation. Therein, dn/dt represents a variation of refractive index due to a temperature variation (e.g., within a range from −20° C. to 40° C.).
The information disclosed in this section is only for enhancement of understanding of the DESCRIPTION OF RELATED ART section of the described technology and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art. Further, the information disclosed in the DESCRIPTION OF RELATED ART section does not mean that one or more problems to be resolved by one or more embodiments of the invention was acknowledged by a person of ordinary skill in the art.